Electric motors are widely used to drive a variety of rotating equipment, such as pumps and other mechanical or electrical devices. Issues considered when installing an electric motor are the horsepower requirement of the motor and the voltage on which the motor operates. Following selection of horsepower and operating voltage of the motor, means of starting the motor is chosen. Selection depends on application requirements, such as compatibility with variable speed control, variable voltage or frequency starting requirements, and limitation of starting capacity, that is, whether the amount of current available on the circuit for starting is limited in any way.
Methods available for starting an electric motor include an “across the line” motor starter, a “variable speed (variable frequency) drive”, a “variable voltage” or “soft starter.” Starter selection is influenced by the requirement that starting an electric motor to rotate from a fully stopped position demands that electric motor windings conduct as much as six to eight times the normal winding operating current, where the windings may be located in the stator, in the rotor, or in both, depending upon the design of the electric motor. This excess demand for current is termed the “lock rotor current” rating of the motor or the instantaneous current draw on the system.
Issues directly related to the lock rotor current rating, especially ratings for large electric motors, may influence the cost of electricity, the installation cost, adequate availability of sufficient capacity from utility distribution circuits, and availability of electric power from the utility. Often a local utility must upgrade power lines coming into an industrial site in connection with installation of large electric motor loads to be able to meet the lock rotor current demands of the electric motor. Costs associated with these issues may prevent installation of large electric motors in some areas.
Most electric utilities in the U.S. consider the availability of extra capacity when establishing an electricity rate to large industrial users of electricity. Electric utilities often penalize large industrial users with higher electricity prices to compensate for peak electricity demands which exceed the base load or constant load requirements of the users. This is especially true when user equipment requires instantaneous and short spikes of current, as when starting an electric motor. Lock rotor current ratings of many electric motors require the utility to have the extra capacity in reserve. Reserve or peak load demand is more expensive to provide in most cases. As a result of the extra cost of using an electric motor to drive equipment, gas engines can be more feasible.
Gas engine-driven compressors are used in most gas compressor stations, despite having significant drawbacks. A portion of the natural gas forwarded at the gas compression station or installation is used to operate the natural gas-fired engines that drive the gas compressors. Because of the high cost of gas-fired engines, gas compressor units usually use high rpm gas engines rather than slower versions. Operation and maintenance costs of the gas engine compressors are usually high and constitute a large portion of the cost of operating a gas compressor. Unscheduled down time due to unexpected engine failures are common. Major engine overhauls are frequently necessary and are costly.
An electric motor driven compressor requiring less maintenance and providing increased run time would be more feasible if the price of electricity were less. However, often, utilities, facing limited capacity in rural areas, force customers to limit their demand for electric power. Accordingly, there is a need to limit or eliminate the lock rotor current draw of electric motors.